This invention relates generally to centrifugal compressors and, more particularly, to an improved lubrication method and apparatus therefor.
Hermetic centrifugal refrigeration compressors generally use an electric motor to drive the impeller through a geared up transmission. In such a compressor, the transmission is typically vented to a source of low pressure refrigerant within the system to minimize the outward migration of oil through the shaft seals. It has been recognized that during this venting process, in addition to the refrigerant gas passing out of the transmission, some of the oil in the form of droplets or mist may become entrained within the refrigerant gas and also pass out from the transmission. This is not been a particular problem.
With the more recent use of higher pressure and higher density refrigerants, such as R-22, the problem of oil carry over has become an issue. That is, because of the higher pressure refrigerant, it is necessary to operate the gears at higher speeds. This is in turn increases the turbulence and the oil mist generation within the transmission. Further, the larger pressure differentials tend to promote higher vent gas flow rates and therefore increased carry over.
In addition to the refrigerant higher pressures, the higher densities also tend to exasperate the problem. That is, the increased densities tend to keep the oil droplets in suspension longer and makes separation more difficult. In addition, the mechanical losses from oil separation mechanisms are higher due to increased density.
Considering now the result of oil carry over, when the oil entrained refrigerant from the transmission is vented to the compressor inlet, it passes through the compressor and is discharged into the condenser, where it may tend to coat the heat exchanger surface to thereby decrease the efficiency thereof. Some of the oil is then passed on to the cooler where the same phenomenon occurs. Thus, it will be recognized that high oil carry over rates tend to result in poor heat exchanger performance. More over, as the oil supply in the sump is diminished because of this phenomenon, there may no longer be sufficient amount of oil to insure that all of the moving parts that require lubrication are in fact receiving adequate supplies of oil.
The oil carry over problem has been addressed in two different ways. First, the most common approach is to use a mesh type oil separator in the vent line to cause oil droplets to coalesce and drain back into the transmission. A second method uses a series of hollow rotating tubes to centrifuge out the unwanted oil mist component of the vent flow. Neither of these methods, by themselves, are found to be sufficient for containing oil in a centrifugal compressor using high pressure, high density refrigerant such as R-22.
In addition to the loss of oil from the transmission as occasioned by the high speed rotation of the gear, there are also mechanical losses brought about by the oil from the bearings being transferred into the gear mesh and onto the gear face. That is, oil on the gear causes windage losses as well as oil pumping losses, since the gear mesh then acts as a pump. These, in turn, increase the load on the bearings and may reduce the life thereof.
It is therefore an objection of the present invention to provide an improved lubrication system for a centrifugal compressor.
Another object of the present invention is the provision in a centrifugal compressor for reducing mechanical losses brought about by oil being disposed on the gears
Yet another object of the present invention is the provision in a centrifugal compressor for reducing windage losses and oil pumping losses in the transmission.
Another object of the invention is to provide a mechanism by which the used oil is conveyed directly to the sump without being "caught up" in the turbulent environment within the transmission.
Still another object of the present invention is the provision in a centrifugal compressor for reducing the load on the bearings.
Yet another object of the present invention is the provision in a centrifugal compressor for a lubrication system which is economical and practical in operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.